Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds such as nitrogen oxides (NOX), and solid particulate matter also known as soot. Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOX and soot emitted to the atmosphere by an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
In order to ensure compliance with the regulation of NOX, some engine manufacturers have implemented an exhaust treatment strategy incorporating SCR. SCR is a process where a gaseous or liquid reductant, most commonly urea or ammonia, is injected into the exhaust gas stream of an engine and is absorbed onto a substrate that has been coated with a reduction catalyst. As the exhaust passes through the substrate, the reductant reacts with NOX in the exhaust gas to form H2O and N2. In general, SCR is most effective when a concentration of NO to NO2 supplied to the reduction catalyst is about 1:1. In order to achieve this optimum ratio, a diesel oxidation catalyst (DOC) is often located upstream of the substrate to convert NO to NO2.
Although SCR with urea or ammonia is useful in some exhaust treatment systems, the use of such reductants can be hazardous. For example, ammonia can cause the direct oxidation of machine components, and the formation of ammonium salts can further corrode such components. Moreover, excess ammonia injected into the exhaust flow upstream of the substrate can often “slip” past the substrate, thus requiring the use of an additional “clean-up catalyst” downstream of the substrate to capture ammonia slip before it is released to the environment. Such clean-up catalysts increase the size, cost, and complexity of the exhaust treatment system.
As an alternative to SCR with urea or ammonia, an SCR process in which hydrocarbons are used as reducing agents may be employed. For example, combustion exhaust produced by natural gas engines and other like combustion engines is principally composed of methane and other hydrocarbons. Such hydrocarbons are capable of acting as reductants in the SCR process under certain conditions, and using such hydrocarbons as reducing agents in the SCR process eliminates the need for carrying a supply of hazardous reductants on the machine.
An exemplary system utilizing the SCR process to treat combustion exhaust is disclosed in U.S. Pat. No. 7,488,462 (the '462 patent). For example, the '462 patent teaches a lean-burn natural gas engine fluidly connected to a catalyst system. The catalyst system includes an oxidation catalyst configured to oxidize NO to NO2. The catalyst system also includes a reduction catalyst configured to reduce NO2 to N2 in the presence of methane and other hydrocarbons present in the exhaust stream.
While the system taught in the '462 patent may be utilized to treat exhaust produced by a natural gas engine, it may be difficult to optimize the efficiency of the disclosed system. For instance, it is understood that increasing the ratio of non-methane hydrocarbons to methane in the exhaust may increase the effectiveness of known reduction catalysts. However, the system taught in the '462 patent does not allow engine operators to increase the proportion of non-methane hydrocarbons in the exhaust. While methane and other hydrocarbons used as reducing agents by the system of the '462 patent may be plentiful in the engine exhaust, these hydrocarbons are easily combusted at elevated exhaust temperatures in the presence of oxygen. As a result of such combustion, a desired amount of hydrocarbons may not be available to sufficiently react with NOx.
The system of the present disclosure solves one or more of the problems set forth above.